The size of an optical mode within a photonic integrated circuit (PIC) is generally much smaller than the optical mode of a connected fibre optic cable. For example, the optical mode within a fibre optic cable may be around 13 μm×13 μm. Whereas the optical mode within the PIC may be typically a few microns or less. This mismatch in optical mode can lead to coupling losses when connecting the PIC to the fibre optic cable.
Generally increasing the mode size of the optical mode within the PIC is not a viable solution, as the resulting optical circuit would be unfeasibly large.
Known in the prior art are mode converters, which convert the optical mode of the fibre optic cable to that of the optical mode within the PIC (and vice versa). Generally, mode converters in the prior art fall within two categories:                (1) Converters which involve modified fibre profiles (for example, lensed or tapered fibres) and active alignment to the PIC via fibres mounted in a fibre block.        (2) Providing a tapered waveguide within the PIC, with integrated v-grooves for passive alignment or fibre attach via a separate fibre block.        
Mode converters falling within category (1) generally demand very tight fibre alignment tolerances, and packaging costs can be high due to the increased number of parts and the labour of precise active alignment of the fibre block to the PIC.
Whereas mode converters falling within category (2) generally result in a large variation in the topography of a PIC, due to the relatively large height of the mode converter in contrast to the remaining components on the PIC. This variation in topography can be challenging when it comes to photolithographic processes used in fabrication as it can degrade the dimensional control of the other components on the PIC. The invention aims to provide a manufacturable method of fabricating a low loss, passively aligned PIC without the topography limitations of the prior art.